Phenyl Isocyanate from Nitrobenzene: New Findings

Carbonylation of nitroarenes constitutes an alternative to the currently employed technology for the production of aromatic isocyanates, which makes use of highly toxic phosgene at one stage. The direct synthesis of aryl isocyanates is the most desirable reaction, but due to the harsh reaction conditions required and the high reactivity of isocyanates themselves, in the last 30 years research has mostly focussed on an indirect route in which carbamates are first produced by performing the carbonylation reaction in methanol as the solvent. Carbamates can be then thermolyzed to isocyanates and alcohol in a second stage.Taking advantage of our experience in the synthesis of carbamates,[1-3] we reconsidered one of the best catalytic systems for the direct synthesis of isocyanates from nitroarenes, based on the use of a palladium catalyst with phenanthroline as a ligand.[4] In this communication, we will describe the results we obtained, which allowed us to increase the substrate/catalyst ratio tenfold, while maintaining high selectivities. Furthermore, both experimental and computational studies provided us a deeper knowledge on the reaction mechanism.